Widely used industrial painting processes mount the articles to be painted on hangers carried on a conveyor and apply a coating of liquid paint onto the cleaned and prepared surfaces of the articles, generally by spraying or immersing the articles in a body of liquid paint. Thereafter the articles are transported by the conveyor through an entrance opening into an oven operated at an internal temperature sufficient to cure the paint on the articles, generally of the order of 350 to 400 degrees Fahrenheit. Each article is maintained within the oven for a sufficient period of time to complete the curing process. The painted articles are then removed from the oven, generally through an exit opening.
Within the oven, the atmosphere is contaminated by vapor liberated from the paint in the curing process, and it is desirable to provide air barriers at the entrance and exit openings of the oven in order to reduce the heat and contamination escaping from the oven into the surrounding atmosphere, and to reduce the energy consumed by the oven. A highly effective air barrier for such industrial ovens is described in U.S. Pat. No. 4,298,341 of the present inventor entitled INDUSTRIAL OVEN HAVING AIR RECIRCULATING MEANS FOR MINIMIZING HEAT LOSS.
The vapor produced by curing of liquid paint constitutes an environment hazard. Within the oven, paint vapor must be restricted to a relatively low level to reduce the risk that the volatile vapor will ignite, and this is accomplished by providing an exhaust from the oven and a fresh air flow into the oven. The exhaust of solvent vapor from the liquid paint curing process is a cost of operation, since it entails loss of heat and disposal of the exhausted vapor, generally to the ambient atmosphere.
The use of powdered paint, rather than liquid paint, to paint manufactured articles reduces the amount of solvent required and also paint required to paint an article. Paint overspray, which is the portion of the paint spray which fails to become attached to an articles in the spraying process, is greatly reduced by the use of powdered paint. Further, the quantity of solvent present in powdered paint is significantly less than that in a comparable body of liquid paint, thus reducing the vapor liberated in curing each processed article.
The painting processes which industry had developed using powdered paint applies a layer of paint in powdered state onto the articles to be painted, and then in the usual manufacturing process, transports the articles on a conveyor line to and through an industrial oven provided with entrance and exit openings with air barrier closures. The conveyor line transports the powder laden articles directly across the air flow of the entrance air barrier. To be efficient, the air barrier requires a relatively high flow rate, and suitable air flow rates for the air barrier are often sufficient to move paint powder about on the surface of the articles and to blow paint powder off of the articles passing through the entrance opening of the oven on the conveyor line, thus producing bare or thin areas of paint or paint buildup on the finished articles. Under some conditions, it has been necessary to reduce the air flow in the entrance air barrier of the oven in order to ameliorate this condition, thus reducing the effectiveness of the air barrier.
The industrial ovens known to the art require a large space, are costly to construct, and have relatively high operating costs, particularly in high production applications. If the oven must heat treat items being produced at a high rate, the oven must be long enough to provide the required residence time for articles moving within the oven at the production rate. Heat is transferred from the environment within the oven, which is an air and vapor mixture, to the process articles largely by contact of the oven environment with the articles. Since the gaseous oven environment of conventional ovens is largely dry air, the oven environment has a low thermal capacity. Heat transfer from the oven environment to the articles in process largely occurs through a film of air on the surface of the article, the film being periodically replaced by circulation of the environment within the oven. This process results in a relatively low rate of heat transfer from the environment of the oven to the precess articles, thus requiring a long path within the oven to provide the desired heat treatment to the articles.
Further, the use of air barriers for closures at the entrance and exit openings of an oven results in a gradual temperature increase from the opening to the interior of oven, thus providing regions within the oven adjacent to the entrance and exit opening of reduced heat transfer. Accordingly, industrial ovens for high production applications are long, resulting in relatively high heat loss and relatively high equipment and building costs.